Exercise devices, and in particular weight training machines, typically include a mechanical member that the user repeatedly moves along a prescribed path for exercise. Conventionally, movement of the mechanical member is resisted in some fashion (often by weights) to render the movement more difficult and thereby intensify the exercise. The movement of the mechanical member determines what muscle or muscle groups are to be involved in the exercise.
One popular exercise movement is the leg curl, which involves bending the legs from an extended position, in which the legs are straight, to a curl position, in which they are bent at the knees at approximately a ninety degree angle. This movement can be performed as the exerciser is seated, standing (this is less common), or, more commonly, as the exerciser is prone. In the prone position, the user is often bent somewhat at the waist (at an angle of between about 5 and 30 degrees). Generally, the leg curl movement exercises the hamstring and, to a lesser extent, the gluteus maximus muscles of the exerciser, and the aforementioned bent position of the exerciser can promote complete contraction of the hamstrings.
Another common exercise movement is the leg extension, which generally can be regarded as the opposing, or reverse, movement of the leg curl. More specifically, the exerciser moves his legs from a position in which the legs are bent at the knees at an angle of approximately ninety degrees to an extended position in which the legs are straight. The leg extension movement, which is typically performed as the user is seated, exercises the quadriceps muscles of the user.
Because the movements are similar, some exercise machines that enable the exerciser to perform either movement have been designed. Such machines can save cost and space; this can be particularly important for multi-station exercise machines, as having a single station that can provide both leg curl and leg extension capability eliminates the need to have either of these movements provided by a different station. One difficulty faced by designers is creating a configuration that properly positions the pivot axes of the leg curl and leg extension movements. Ideally, the pivot axis of the curl and extension movement arms would be coincident with the pivot axis of the knee joint, which is typically at a location just inside the dorsal surface of the knee when the leg is extended. If these pivot axes are not properly positioned, the exercise can place stress on the knee joint.
One exemplary machine is the PERSONAL CIRCUIT machine, available from Nautilus HPS, Inc., which is designed for a seated exerciser for both movements. This machine has two movement arm positions: one for the leg curl movement that is at rest at approximately the height of the seat; and another for the leg extension movement that is at rest in a position directly below the front edge of the seat (typically, the movement arm is repositioned relative to other parts of a single overall assembly). However, this type of machine requires that the resistance to the movement arm motions be directed upwardly for the leg curl movement and downwardly for the leg extension movement. As such, this machine employs some additional components (pulleys, cables, cams, or the like) that can redirect the direction of resistance in order for its single weight stack to provide resistance for both movements; these additional components typically add cost to the machine. In addition, there can be a reduced range of motion for the leg curl for a seated leg curl. Further, when the exerciser is performing a seated leg curl, the weight of the lower leg itself assists, rather than resists, the movement, thereby requiring that the machine provide a higher load for the same degree of exercise resistance.
Another leg curl/leg extension machine configuration is exemplified in U.S. Pat. No. 5,002,271 to Gonzales. In the machine illustrated therein, a hinged bench has a generally horizontal seat and generally vertical backrest for a leg extension movement; these convert to a flat horizontal platform for a prone leg curl movement. The machine includes a single assembly for the leg curl and leg extension arms that is pivoted at the front edge of the “seat”. This pivot location is, for the reasons set forth above, a biomechanically disfavored location, as it fails to align the exerciser's knee joint with the exercise arm pivot axis for both the leg curl and leg extension movements. A similar device is illustrated in U.S. Pat. No. 5,042,801 to Sterba et al.